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Where have the salmon gone?

When the expected Sockeye salmon run of eight million in the Fraser River fell to an alarming 600,000, many suspected habitat mismanagement, over-fishing, climate change, and even the state of health of the fish themselves.

Graduate student Melissa Evans is hoping to find the answer. Her PhD project - studying the immunological health within the Chinook salmon populations of coastal British Columbia – has turned up some interesting variables.

Along with PhD supervisor Bryan Neff, Evans examined the genetic variation of the Major Histocompatibility Complex (MHC) within salmon populations. The MHC is a group of genes that code for proteins, which are expressed on the surface of cells.

These proteins help the immune system to recognize bacteria, viruses, and other infectious agents. This recognition is the first step in mounting a specific defense against these diseases.

Evans examined kidney samples from salmon fry for evidence of bacterial infections, looking at the relationship between MHC genetic variation and the bacteria that were infecting the fish. She noticed the paired chromosomes, which carried two different copies of the genes, gave an advantage to the fry within the population maintaining this genetic variation.

Another finding revealed certain genetic variants would be more susceptible to certain bacterial infections. Although a preliminary result, it is fodder for future investigative trials.

“That [severe population declines] could play an important role in the next generation’s genetic variation and their ability to potentially be resistant to infections by parasites,” says Evans. “It might be important to their survival.”

Evans adds her research opens up speculation on many questions involving the aftermath of declining fish stocks. Until now, there was no basis for a “big picture” of infection in salmon populations.

“Our study is one of the first to attempt to look at the bacterial community as a whole,” says Evans. “Most studies look at one parasite they think is important, they don’t necessarily know what’s going on at the ecological interaction level.”

This interaction could be substantial in shaping the future of salmon populations, she adds. For Sockeye salmon, emerging infectious diseases are a potentially important factor driving mortality within populations.”

“Relatively little is known about what is going on in the ocean, but we do know that in the fresh water individuals are being faced with a number of parasites that may be causing increased mortality.”

The extent of the effects of those parasites on the population is unknown right now.

“They did not predict these populations were going to come back in such low numbers,” says Evans. “But certainly we know there are parasites out there that are affecting juveniles and spawning adults.”